The Technical Brief AC Reverse Polarity describes why and in what cases Reverse Polarity Indicators are required by ABYC in shipboard AC systems. This Technical Brief discusses the detailed requirements and workings of the Reverse Polarity Indicator (RPI) itself. It is suggested that the reader be familiar with the Technical Brief AC Reverse Polarity before reading this one.

Perhaps the easiest way to visualize the working of an RPI is to first visualize the relationship of the Safety Ground and Neutral wires. In a normal marine installation, when connected to shore power, these two wires are are not connected on the boat, but are connected together on shore at the system grounding point.

The job of the RPI is to determine if there is voltage potential between Safety Ground and Neutral. By looking at the diagram is easy to see that in a proper installation Safety Ground and Neutral are effectively two ends of the same wire and therefore should have the same voltage potential and not be capable of lighting the RPI when installed as shown below:

This also explains the two notes about RPI's in the ABYC Standards:

Reverse polarity indicating devices respond to the reversal of an ungrounded conductor and the grounded (white) conductor only when there is continuity of the grounding (green) conductor to shore.

Reverse polarity indicating devices might not respond to reversals of an ungrounded conductor and the grounding (green) conductor, the grounded (white) conductor and the grounding (green) conductor, or three phase conductors.

One further ABYC requirement for RPI's is that they contain a minimum 25,000 OhmResistor. A full treatment of the reason for this is beyond the scope of this Technical Brief. It is sufficient, however, to say that because the Safety Green wire is connected to devices aboard the boat that may contain stray currents, the link created by an RPI could provide a path for stray currents via the grounded Neutral wire.